Integrated power transistor circuits comprise a multiplicity of transistor cells and are often provided with current-measuring cells, which enable the measurement of the load current. In general, for this purpose, one of the transistor cells is selected as a current-measuring cell (sense transistor cell) and operated substantially in parallel with the other transistor cells i.e. the power transistor cells. While the drain electrodes and gate electrodes of the current-measuring cell and the power transistor cells are each driven jointly, the potentials at the source electrodes of the current-measuring cell and the power transistor cells are detected separately. The load current flowing through the power transistor cells can be estimated from the difference between the source potentials of the current-measuring cell and the power transistor cells.
It is desirable to measure the source potential of the power transistor cells as accurately as possible without losing active chip area for the transistor cells and the electrodes thereof.